Metal nanocrystals and metamaterials with strong plasmonic resonances in the visible and infrared spectral intervals often exhibit unique optical properties. In single nanocrystals with specially designed shapes, plasmonic resonances can be efficiently tuned with the geometry of the nanocrystals and can be made strong and narrow. These properties are characteristic for plasmonic nanorods that exhibit strong and narrow longitudinal resonances. Whereas nanospheres, nanorods, and nanocubes with small sizes can be grown using a colloidal synthesis in solution, larger-size nanostructures are conveniently fabricated by lithographic methods. Lithographically made 2D and 3D metamaterials employ electromagnetic interactions between building blocks to create interesting optical responses.
One of the prominent effects originating from the interactions between single nanocrystals is the Fano effect. This effect can occur in a purely plasmonic system or in hybrid exciton-plasmon nanostructures. The Fano effect typically originates from an interaction between broad and narrow resonances in a system composed of two or more elements. Another phenomenon, related to the plasmonic Fano effect, is the plasmon-induced transparency of planar metamaterials composed of a small number of interacting nanocrystals. This plasmon-induced transparency manifests as a localized maximum in a transmission plot, which generally also corresponds to a transparency window in an absorption plot. Additionally, spectral windows may be formed through an extraordinary optical transmission in plasmonic nanohole arrays. The transmission windows are attributed to the presence of constructive interference of surface plasmonic waves. The extraordinary optical transmission is an angle-dependent effect.
Prior to the present invention, isotropic optical materials with a broad extinction spectrum featuring a narrow transparency window were unknown. Such nanomaterials could be useful for smart coatings and screens for shielding electromagnetic radiation. Known planar nanostructures, which do exhibit the window effect, are anisotropic and exhibit interference effects. Planar Bragg reflectors and metamaterials exhibit strongly anisotropic transmission spectra. Additionally, Bragg-reflector filters and planar metamaterials featuring transmission bands are based on electromagnetic interference and interactions between their elements. However, designing a medium or metamaterial capable of attenuating light in a broad spectral interval while simultaneously exhibiting a narrow transparency window at a given wavelength has proven challenging.